1. Field of the Invention
The present invention relates to the field of computed tomography (CT) scan, and more specifically to a method for generating a bone mask by utilizing the images from the standard nonenhanced CT in axial mode scan.
2. Description of Related Art
Being a well-established and fully developed technique, computed tomography angiography (CTA) can be used for the detections of cerebral aneurysms, artery stenoses and other vascular anomalies. Comparing to the conventional angiography, this technique is less invasive. Furthermore, comparing to the magnetic resonance angiography (MRA), CTA has the advantage of having a shorter examination time, a lower cost and loosened screening criteria for patient suitability. Therefore, in the acute setting, CTA is performed frequently for diagnosing vascular diseases. Although three-dimensional visualization and computer-aided analytic techniques such as maximum intensity projection (MIP), volume rendering and arterial lumen analysis are readily available, fully automated image processing for CTA data in the brain has been hampered by the presence of surrounding bony structures, particularly in the region of the skull base. For this reason, radiologists must resort to a time-consuming fundamental 2-dimensional sectional image evaluation for vessels surrounded by bone.
When the value of Hounsfield unit (HU) for the contrast-enhanced blood vessels following the injection of contrast medium falls within the HU range of the bone and the calcification, diagnosis by computed tomography angiography becomes more difficult. Radiologists and radiation physicists have already proposed many methods to remove bone structures or extract out vessels from the CTA source images, including using subtraction CTA, manual or automated bone editing, matched mask bone elimination (MMBE), vessel segmentation and dual energy CTA.
Subtraction CTA is the simplest method, this involves using the data acquired from the additional, precontrast helical computed tomography (CT) scan as a reference. This reference was then subtracted from the data acquired from the postcontrast helical CT scan to produce contrast-enhanced images. However, subtraction CTA has the following two shortcomings: (1) the radiation dose in which the subject was exposed to, doubled as the result of the additional precontrast helical CT scan; (2) the angiography images with added noise (caused by subtraction operations). Conversely, even though bone editing and vessel segmentation methods do not require any additional, precontrast CT scan, these methods involves drawbacks of having a certain degree of computational complexity and having significant differences in results edited by different operators.
Matched mask bone elimination (MMBE) is currently the most common approach being used, which also requires an additional, precontrast helical CT scan in order to acquire the data for bone structure elimination. Compared with subtraction CTA, the advantage of using this approach is that the additional scan can be carried out by using only a quarter of the standard radiation dose. This technique generates a bone mask by simple thresholding on the low dose precontrast data, and then eliminates the bone in the data of the postcontrast helical CT scan by matching the bone mask back. Evidence from previous studies also points out that CT angiography with matched mask bone elimination is a technique that accurately detects intracranial aneurysms.
In a standard clinical procedure today, patients with symptoms of acute stroke would first undergo a noncontrast CT scan to determine the types of stroke: cerebral infarction, primary intracerebral hemorrhage or subarachnoid hemorrhage. Although multidetector CT can generate noncontrast helical CT images that are as good as axial CT images in terms of quality, the noncontrast CT scan is constantly done in axial mode to avoid helical scanning artifacts. Therefore, this invention provides a method for generating a bone mask by utilizing the images from the standard nonenhanced CT in axial mode scan. When a patient had received the nonenhanced CT in axial mode scan for diagnosis, the resulting data from the scan can be used repeatedly to generate the bone mask required for helical CT angiography, thereby reducing the total radiation dose that a patient exposed to.